In recent years, as an information recording medium, optical disks such as CDs (Compact Disks) and DVDs (Digital Versatile Disks) have become predominant. In a reproducing device of the optical disks, an optical pickup unit irradiates laser light along a track of an optical disk and detects reflected light thereof. Based on a variation in intensity of the reflected light, recorded data are reproduced.
An optical disk reproducing device, while detecting data based on the reflected light, servo-controls a positional relationship between the optical pickup unit and the optical disk. Specifically, a tracking servo that allows irradiation of the laser light along a centerline of the track and a focusing servo that can keep a distance between the optical disk and the optical pickup unit constant are implemented. For instance, in the focus servo control, based on an output signal of the photodetector that detects reflected laser light, an actuator variably controls a position of the optical pickup unit to maintain a distance d with the optical disk constant. As a result, an amount of reflected light corresponding to a displacement of a focus of irradiation light on a surface of the optical disk can be prevented from fluctuating and thereby noise superposed on the light receiving signal can be suppressed.
In order to obtain information for such servo control, as a photodetector, a semiconductor device where a reflected light image is divided into a plurality of segments and received is used. FIGS. 1 through 3 are schematic diagrams showing a light receiving portion of the photodetector and a reflected light image on the light receiving portion. Reflected laser light is input on the photodetector through a cylindrical lens. The reflected light has a circular section when it enters the cylindrical lens. According to a principle of an astigmatism method, an image of the reflected light after going through the cylindrical lens, in accordance with a distance d between the optical pickup unit and the optical disk, varies in a dimensional ratio in two perpendicular directions. Specifically, when a distance d is a target value, as shown in FIG. 2, an image of reflected light is set so as to be a perfect circle 30. On the other hand, for instance, when the distance d is excessive, as shown in FIG. 1, an image of reflected light becomes a vertically long ellipse 32 and, when the distance d is insufficient as shown in FIG. 3, an image of reflected light becomes a horizontally long ellipse 34.
The photodetector has a light receiving portion that is divided into 2×2=4 segments 36 and each of the segments constitutes a light receiving element that outputs a light receiving signal. The photodetector is arranged so that diagonal directions of a 2 ×2 square arrangement of the light receiving elements, respectively, may coincide with axes of the vertically long ellipse 32 and the horizontally long ellipse 34. When the light receiving elements are thus arranged, in FIGS. 1 through 3, based on a difference between a sum of output signals of two light receiving elements arranged on a diagonal line along a vertical direction and a sum of output signals of two light receiving elements arranged on a diagonal line along a horizontal direction, shapes of the reflected light image as shown in FIGS. 1 through 3 can be distinguished. The shape of the reflected light image can be used to control the distance d. On the other hand, the intensity of light reflected in accordance with data can be obtained from a total sum of output signals of four light receiving elements.
Since a data rate read from an optical disk is very high, the photodetector is constituted of a semiconductor device that uses a PIN photodiode having high response speed. FIG. 4 is a schematic sectional view of an existing photodetector. The drawing expresses a sectional view that goes through two adjacent light receiving elements and is vertical to a semiconductor substrate. The semiconductor device has a P+ region that becomes an anode region 42, which is formed on a surface of a P-type semiconductor substrate 40. Above the anode region 42, an i layer 44 that has a low impurity concentration and high resistivity is formed by an epitaxial growth method. In the i layer 44, at a position corresponding to a boundary of the light receiving elements, an isolation region 46 that is made of a P+ region and continues to the anode region 42 is formed. Furthermore, on a surface of the i layer 44, an N+ region that becomes a cathode region 48 is formed.
The anode region 42, the i layer 44 and the cathode region 48 constitute a PIN photodiode that becomes a light receiving element of a photodetector. The anode region 42 and the cathode region 48, respectively, are connected to voltage terminals and a reverse bias voltage is applied therebetween. In a reverse bias state, in the i layer 44 between the anode region 42 and the cathode region 48, a depletion layer is formed and electrons generated in the depletion layer owing to absorption of incident light move to the cathode region 48 owing to an electric field in the depletion layer, followed by outputting as a receiving light signal. Here, the isolation region 46, as mentioned above, reaches the anode region 42 from a surface of the i layer 44. As a result, the i layer 44 is divided for every light receiving element thus making it possible to inhibit crosstalk between light receiving elements.
A thickness of the i layer 44 is set equal to or more than a substantial absorption length of detecting light in a semiconductor. For instance, an absorption length of silicon to light of a 780 nm or 650 nm band that is used in, for instance, a CD or DVD is substantially 10 to 20 μm. The P+ layer of the isolation region 46 is formed, after the ion implantation, by pressing in a depth direction by means of thermal diffusion. However, at that time, in the thermal diffusion, the P+ region is expanded not only in a depth direction but also in a horizontal direction. In this connection, when the i layer 44 is relatively thick, in order to form an isolation region 46 that is restricted in width, the i layer 44 is formed divided into a plurality of times of epitaxial growth. In this case, every time an epitaxial layer 50 is formed, the ion injection and thermal diffusion are carried out from a surface thereof and thereby an isolation region 52 reaching a bottom surface of the epitaxial layer 50 is formed. When the epitaxial layers 50 and isolation layers 52 are thus layered, the isolation region 46 extending in a depth direction can be formed with a width that is prevented from expanding.
In a semiconductor device that constitutes an existing photodetector, a cathode region 48 is disposed on a surface of a semiconductor substrate, an i layer 44 located below the cathode region 48 forms a depletion layer, and in the depletion layer signal charges are generated by photoelectric conversion. In this configuration, there is a problem in that it is difficult to detect light of an absorption length that is substantially a thickness of the cathode region 48 or less, namely, relatively short in wavelength, for instance, blue light, being absorbed in the cathode region 48. The problem becomes particularly important when an optical disk reproducing device compatible with short wavelength light capable of improving the recording density is being realized.
Furthermore, in a semiconductor device that constitutes an existing photodetector, when a relatively thick i layer such as 10 to 20 μm is formed, the formation of an epitaxial layer 50 and an isolation layer 52 is repeated a plurality of times. Accordingly, there is a problem in that a semiconductor device is high in manufacturing cost. There is a further problem in that, for a part of a junction area of the isolation region 46 and the i layer 44, a capacitance between terminals of an anode and a cathode increases and as a result the high speed responsiveness that is a feature of the PIN photodiode is damaged.
The invention intends to provide a semiconductor device capable of detecting a short wavelength light component and reducing the manufacturing cost and having responsiveness suitable as a photodetector that detects a light signal from an optical disk and so on.
[Patent literature 1] JP-A-10-107243
[Patent literature 2] JP-A-2001-60713